The dramatic importance of natural killer (NK) cell activity to spontaneous leukemia incidence was demonstrated by converting the low-NK activity, high spontaneous-leukemia AKR strain of mice to high-NK activity and very much lower leukemia incidence by adoptive transfer of bone marrow from the high-NK, low-leukemia (C57xAKR)F1 hybrid, as contrasted to the low-NK, high-leukemia (C3HxAKR)F1 hybrid donor.